The present invention relates to a variable displacement swash plate type compressor that constitutes part of, for example, a refrigerant circuit for a vehicle air conditioner and is configured to change the displacement by changing the pressure in a control pressure chamber to change the inclination angle of a swash plate.
A variable displacement swash plate type compressor has a bleed passage, which extends from a control pressure chamber to a suction pressure zone, and a supply passage, which extends from a discharge pressure zone to the control pressure chamber. A control valve controls the pressure in the control pressure chamber, so that the inclination angle of a swash plate is changed. This reciprocates pistons engaged with the swash plate by a stroke corresponding to the inclination angle of the swash plate, so that the displacement is changed. The control valve controls the amount of refrigerant gas to be supplied from a discharge pressure zone via the supply passage to the control pressure chamber by controlling the opening degree of the supply passage. Refrigerant gas is discharged from the control pressure chamber via the bleed passage to the suction pressure zone, so that the pressure in the control pressure chamber is controlled.
Such a variable displacement swash plate type compressor constitutes part of a refrigerant circuit (cooling circuit) for a vehicle air conditioner. The refrigerant circuit is provided with a variable displacement swash plate type compressor and an external refrigerant circuit. The external refrigerant circuit includes a condenser, an expansion valve, and an evaporator. A discharge chamber of the variable displacement swash plate type compressor and the inlet of the condenser are connected to each other via a discharge passage. The outlet of the evaporator and a suction chamber of the variable displacement swash plate type compressor are connected to each other via a suction passage. A restrictor, e.g., a fixed restrictor, is provided at the middle of the discharge passage. The restrictor lowers discharge pulsation of refrigerant gas.
In a vehicle, compressor driving torque required for driving a variable displacement swash plate type compressor, which uses the engine as a drive source, is estimated in order to suitably control the engine output. In general, the displacement is used as a parameter for estimating the compressor driving torque. Thereupon, a differential pressure is detected between a pressure (PdH) at a first pressure monitoring point, which is located on the upstream side of the restrictor in the discharge passage in the flow direction of refrigerant gas circulating through a refrigerant circuit, and a pressure (PdL) at a second pressure monitoring point, which is located on the downstream of the restrictor in the discharge passage. This differential pressure will be hereinafter referred to as “a point-to-point differential pressure”. A control valve, which is provided with a differential pressure detecting means for applying a load based on the point-to-point differential pressure to a valve body, is disclosed in Japanese Laid-Open Patent Publication No. 2001-221158, for example.
The differential pressure detecting means is connected to and driven by a flow rate setting means. The flow rate setting means applies an urging force that counters the load applied to a valve body by the differential pressure detecting means based on a point-to-point differential pressure, and sets a target value of the flow rate of refrigerant in a refrigerant circuit in accordance with the urging force. The flow rate setting means is provided with an electric drive unit (solenoid portion), which is configured to change the urging force when being electrically controlled from outside. By electrically controlling the electric drive unit, the opening degree of the valve body is controlled in a state where there is equilibrium between the load applied to the valve body by the differential pressure detecting means based on the point-to-point differential pressure and the urging force applied to the valve body by the flow rate setting means to the valve body.
As the flow rate of refrigerant gas flowing through the restrictor becomes higher, the point-to-point differential pressure becomes larger. As the flow rate of refrigerant gas flowing through the restrictor becomes lower, the point-to-point differential pressure becomes smaller. Accordingly, the point-to-point differential pressure has a correlation with the flow rate of refrigerant gas flowing through a restrictor, i.e., the flow rate of refrigerant flowing in the refrigerant circuit. The flow rate of refrigerant gas flowing through the restrictor is equal to the displacement of the variable displacement swash plate type compressor. This enables determination of the displacement of the variable displacement swash plate type compressor, such as the compressor described in the above publication, provided with a control valve by directly measuring the supply amount of electricity to the solenoid portion, which is correlated to the displacement. Accordingly, it is possible to estimate the compressor driving torque using the displacement, without providing a flow rate sensor, for example, for detecting the flow rate of refrigerant gas.
In a variable displacement swash plate type compressor having single-headed pistons, a swash plate chamber functions as a control pressure chamber in order to change the inclination angle of the swash plate. A load based on the point-to-point differential pressure acts on a valve body and thus the opening degree by the valve body in a supply passage is maximized in a state where electricity supply to the solenoid portion is at a stop, for example. Accordingly, the supply amount of refrigerant gas from the discharge pressure zone via the supply passage to the swash plate chamber is maximized. This minimizes the inclination angle of the swash plate and thus minimizes the displacement of the variable displacement swash plate type compressor.
In contrast, when electricity is supplied to the solenoid portion, urging force applied to the valve body by the solenoid portion to the valve body acts on the valve body, and thus the opening degree by the valve body in the supply passage becomes larger than the maximum degree. Accordingly, the supply amount of refrigerant gas from the discharge pressure zone via the supply passage to the swash plate chamber is decreased, and thus the inclination angle of the swash plate is increased. Accordingly, the displacement of the variable displacement swash plate type compressor is increased.
The solid line in the graph of FIG. 20 is a characteristic line L1 illustrating the relationship between the point-to-point differential pressure generated by a restrictor having a certain passage cross-sectional area (restrictor diameter) and the flow rate of refrigerant gas. As illustrated in FIG. 20, the differential pressure between a first pressure monitoring point and a second pressure monitoring point via a restrictor is unlikely to be generated in a region where the flow rate of refrigerant gas is small. That is, fluctuation in the point-to-point differential pressure is small with respect to fluctuation in the flow rate of refrigerant gas. Accordingly, in a region where the flow rate of refrigerant gas is small, it is required to slightly change urging force applied to the valve body by the solenoid portion in the process of controlling the opening degree of the valve body by the solenoid portion. This makes it difficult to control the displacement of the variable displacement swash plate type compressor.
As the displacement increases, the pressure in the discharge pressure zone becomes higher. Accordingly, an increase in the displacement increases the differential pressure between the pressure in a discharge pressure zone and the pressure in a suction pressure zone (hereinafter referred to as “DS differential pressure”). That is, the DS differential pressure has a correlation with the flow rate of refrigerant gas. Especially in a variable displacement swash plate type compressor having single-headed pistons, fluctuation in the pressure in a swash plate chamber with respect to fluctuation in the displacement is approximate to fluctuation in the pressure in the suction pressure zone. This makes the differential pressure between the pressure in the discharge pressure zone and the pressure in the swash plate chamber (hereinafter referred to as “DC differential pressure”) larger as the displacement increases. That is, the DC differential pressure has a correlation with the flow rate of refrigerant gas as well.
Thereupon, assume a case where a load based on the DC differential pressure is caused to act on the valve body in the same direction as the direction of the load applied to the valve body based on the point-to-point differential pressure, for example. In such a case, in the process of controlling the opening degree of a valve portion by the solenoid portion in a region where the flow rate of refrigerant gas is small, fluctuation in the flow rate of refrigerant gas with respect to fluctuation in the point-to-point differential pressure is unlikely to occur since the load based on the DC differential pressure acts on the valve body. As a result, fluctuation in the flow rate of refrigerant gas with respect to fluctuation in the point-to-point differential pressure becomes smaller in a region where the flow rate of refrigerant gas is small. This improves controllability of the displacement of the variable displacement swash plate type compressor in a zone where the flow rate of refrigerant gas is small.
In contrast, in a double-headed piston swash plate type compressor, a swash plate chamber cannot function as a control pressure chamber for changing the inclination angle of a swash plate as in a variable displacement swash plate type compressor having a single-headed piston. Thereupon, a compressor provided with an actuator that changes the inclination angle of a swash plate is disclosed in Japanese Laid-Open Patent Publication No. 1-190972, for example.
The actuator has a partition body, which is provided on a rotary shaft, a movable body, which moves in a swash plate chamber in a direction along the rotational axis of the rotary shaft, and a control pressure chamber, which is defined by the partition body and the movable body. The control pressure chamber moves the movable body by introducing refrigerant gas from the discharge pressure zone. Introduction of refrigerant gas into the control pressure chamber changes the internal pressure of the control pressure chamber and thus moves the movable body in the axial direction of the rotary shaft. As the movable body is moved along the axis of the rotary shaft, the inclination angle of the swash plate is changed.
Specifically, as the pressure in the control pressure chamber becomes higher and the pressure in the control pressure chamber approaches the pressure in the discharge pressure zone, the movable body moves toward an end of the rotary shaft in the axial direction. The movement of the movable body increases the inclination angle of the swash plate. As the pressure in the control pressure chamber becomes lower and the pressure in the control pressure chamber approaches the pressure in the suction pressure zone, the movable body moves toward the other end of the rotary shaft in the axial direction. The movement of the movable body decreases the inclination angle of the swash plate. As the inclination angle of the swash plate is reduced, the stroke of the double-headed pistons is reduced. Accordingly, the displacement is decreased. Therefore, as the inclination angle of the swash plate increases, the stroke of the double-headed piston becomes larger and the displacement increases.
In a variable displacement swash plate type compressor that uses an actuator for changing the inclination angle of a swash plate, the pressure in the control pressure chamber largely fluctuates between the pressure in the suction pressure zone and the pressure in the discharge pressure zone with fluctuation in the displacement as in the double-headed piston swash plate type compressor. That is, it is difficult to obtain a correlation of a differential pressure (DC differential pressure) between the pressure in the discharge pressure zone and the pressure in the control pressure chamber with fluctuation in the displacement. This makes it difficult to improve controllability of the displacement of the variable displacement swash plate type compressor in a region where the flow rate of refrigerant gas is small, even by causing the load of the DC differential pressure to act on the valve body as described above in the same direction as the direction of the load applied to the valve body based on the point-to-point differential pressure.